A high efficiency motor, which can utilize magnet torque as well as reluctance torque by mounting an interior magnet into a rotor core, is well known in the market, one example of which is disclosed in the Japanese Patent Application Unexamined Publication No. H08-331823. FIG. 15 is a cross section illustrating this kind conventional motor.
In FIG. 15, a stator 1 comprises a plurality of teeth 11 and yokes 12 which connects roots of the plural teeth, and the stator 1 is shaped like a ring. A plurality of slots 13 formed between the teeth are wound with a three-phase winding. A rotor 7 is substantially coaxial with the stator 1 and is shaped like a cylinder. The rotor 7 has four rotor poles facing the inner face of stator 1, and is supported by a bearing (not shown) so that the rotor 7 can rotate on a shaft 24. Four slits 72 punched through axially and disposed at an equal interval along a rotational direction of the rotor core 71 are provided on the rotor 7, and a plate-like permanent magnet 73 is inserted into each slit. A terminal plate (not shown) is disposed on each axial end of the rotor core 71 to cover the permanent magnet 73. The terminal plate is fixed on the end face by riveting a pin 26 through a hole 25, whereby the permanent magnet 73 is fixed into the rotor core 71. An outer circle of the rotor 7 has a notch 77 at a boundary area between the rotor poles, and both of longitudinal ends of the permanent magnet 73 are adjacent to the notch 77. An electric current runs through the stator coil to form a rotating magnetic field. Then, the rotor poles attract/repel the teeth 11 of the stator 1, whereby the rotor 7 is rotated.
In the above structure, the following relation is established between inductances Ld and Lq: EQU Ld&lt;Lq
where Ld is an inductance along "d" axis which crosses the rotor pole at a right angle, and
Lq is an inductance along "q" axis runs through the boundary area between the rotor poles. PA1 Pn: a number of paired rotor poles, PA1 .phi.a: interlinkage magnetic flux PA1 I: stator coil current PA1 .beta.: leading phase angle of the current I (electrical angle)
In general, motor torque T is expressed by the following equation: EQU T=Pn{.phi.a.multidot.I.multidot.cos .beta.+0.5(Lq-Ld)I.sup.2 .multidot.sin 2.beta.} (1)
where,
In the equation (1), the first term represents magnet torque, and the second represents reluctance torque. Since the relation of Ld&lt;Lq is satisfied, a winding current is so controlled to advance the phase of the winding current I with regard to respective induced voltage generated in each phase winding, thereby .beta. becomes greater than zero (.beta.&gt;0), and the reluctance torque is generated. When .beta. is set at a predetermined value, the greater torque T can be produced with a same current than the case where only the magnet torque is available.
However, according to the above structure, since a steel portion 78 having a high permeability exists between the slit 72 and the notch 77, magnetic flux at the longitudinal end of the permanent magnet 73 runs through Pa (magnetic path of the steel portion 78) and is short-circuited as shown in FIG. 15, although it would be expected to reach the stator 1 and contribute to generate the torque. In other words, the magnetic flux decreases by the short-circuited amount, thereby lowering a motor efficiency. Further, the magnetic flux resulted from short-circuited increases cogging torque, which makes a motor noisy and vibrational.